1. Field of the Disclosure
The present invention relates generally to power supplies and, more particularly, the present invention relates to forward converters.
2. Background
AC-to-DC and DC-to-DC power supplies typically use a power conversion topology commonly known in the art as a forward converter.
A forward converter may use either one or two active switches to apply an input voltage to the primary winding of a transformer. The single-switch forward converter uses one active switch to apply an input voltage to the primary winding of a transformer. The two-switch forward converter uses two active switches to apply an input voltage to the primary winding of a transformer. In each type of forward converter, a secondary winding on the transformer produces a scaled replica of the voltage on the primary winding. The voltage on the secondary winding is rectified and filtered to become an output voltage.
In a power supply, the output voltage is normally regulated by a control circuit. The control circuit compares the output voltage to a desired value. The control circuit turns the active switches on and off, and adjusts the time that the switches are on (and off) to keep the output near the desired value.
The choice of one or two switches in the design of a forward converter is heavily influenced by cost. The two-switch forward converter is often the lowest cost configuration that meets the requirements of power supplies for personal computers and similar applications.
Both the single switch configuration and the two switch configuration allow the magnetic flux of the transformer to reset (that is, return to a much lower value) when the active switches are off. Resetting the magnetic flux of the transformer prevents excess stored energy from saturating the transformer (which alters properties of the transformer). The reset is generally achieved by applying a reset voltage of appropriate magnitude and duration to the primary winding when the active switches are off.
It is often desirable to set the reset voltage to a higher value than the input voltage that appears on the primary winding when the switches are on. A common low-cost technique to provide a suitable reset voltage uses a simple reset circuit to develop a substantially constant voltage that is applied to the primary winding during the reset time of the transformer. In a two-switch forward converter, the reset voltage is the sum of the input voltage and the voltage of the reset circuit. In a single-switch forward converter, the reset voltage is the voltage of the reset circuit.
A difficulty with the technique that uses the simple reset circuit is that the appropriate reset voltage can change suddenly when the control circuit responds to a change in the input voltage or to a change in the load on the power supply. Also, the simple reset circuit usually cannot respond fast enough to transient events (such as the start-up and the shut-down of the power supply) to guarantee a proper reset of the transformer.